In the processing and handling of aggregate materials such as sand, gravel or crushed stone, as well as in related industrial materials like coal, slag, iron ore, phosphate, potash, primary metal and related chemical industries, it is necessary to utilize relatively large quantities of water or other liquids in conjunction with or as a dispersing medium for finely sized solid particles produced in the respective grading, concentration or other process. At some point in the process, it is usually necessary to subsequently effect a separation or dewatering of these fine solid materials from the slurry containing them.
One method of dewatering fine granular material prior to disposing of the waste water or other liquid has been to subject the mixture to suitable dewatering devices. The most widely employed method for dewatering in the mineral aggregates industry is an inclined screw dehydrator which slowly moves the solid material up the incline of the screw thread out of a feed basin permitting back flow of the water to waste. However, such equipment has limited water handling capacity and is plagued by the loss of valuable fines which are carried away in the back flow. Also, while such equipment only consumes an amount of energy, typically of 15 horsepower to dry 100 ton per hour to a level of 20% by weight to 25% by weight moisture, the moisture level remains high.
Another technique involves the use of centrifugal force to remove the free moisture. However, the high cost, high power consumption, and wear characteristics associated with such a centrifuge apparatus have prevented the wide use of that technique on a commercial basis for handling abrasive materials. Similarly, the use of pressure or vacuum filters has not been commercially attractive, particularly for the sand and gravel and crushed stone industries.
Another common technique employed for dewatering fine particle slurries is the use of vibrating dewatering screens. The deck of these screens have often taken the form of finely woven wire cloth through which material may pass. Some have in recent times used urethane decks with small openings for water to pass. However the typical horsepower consumption of a dewatering screen system is high, i.e., 85 horsepower per 100 ton per hour, to achieve a moisture level of 8% by weight to 13% by weight; the typical vibratory dewatering screw, while it does dry to levels of moisture of from 8% by weight to 13% by weight, uses an 85 horsepower per 100 ton per hour power consumption. Moisture content may vary by particle size and mineral composition. The assignee of the current application manufactures both conventional dewatering screws and dewatering screens. As discussed, both have deficiencies, either high residual moisture or high power consumption to achieve low moisture.
As can be appreciated, the industry is constantly seeking improved methods for dewatering large quantities of fine solid slurries, particularly sand and gravel but not exclusively sand and gravel. Moreover, the industry is also seeking this improvement in ways which decrease moisture level efficiently with minimum power consumption.
Accordingly, it is a principle object of the present invention to provide a new and improved dewatering device which combines the advantages of an inclined rotatable screw and a vibratory dewatering screen, each designed so that they will cooperate together, i.e., co-act and provide a new and improved dewatering system that substantially increases the efficiency of the dewatering operation to achieve low moisture levels at lower power consumption cost.
Another object of the present invention is to provide a method of dewatering which achieves the above objective without the need of employing impractical and cost inefficient techniques such as centrifugal apparatus, pressure or vacuum filters, etc. The present device and method is uniquely suited, particularly for the sand and gravel and crushed stone industry, to provide lower cost effective dewatering.
A better understanding of the objects, advantages, features, properties and relationships of the component parts and the entirety of the invention will be obtained from the following detailed description and accompanying drawings which set forth an illustrative preferred embodiment and are indicative of the way in which the principles of the invention can be employed.